Published April 1, 1925
STUDIES IN EXPERIMENTAL SYPHILIS.
I. THE INFLUENCE OF THE SIZE OF INOCULUM ON THE COURSE OF
EXPERIMENTAL SYPHILIS IN THE RABBIT.
BY ALAN M. CHESNEY, M.D., AND JAROLD E. KEMP, M.D.
(From the Department of Medicine of the Johns Hopkins MedicalSchool, Baltimore.)
(Received for publication, November 19, 1924.)
INTRODUCTION.
The question of the influence of size of inoculum upon the course of experimental
syphilis in the rabbit has received practically no attention from workers in this
field. Neisser' concluded from his studies of experimental syphilis in monkeys
that the number of treponemata in the inoculum was themostimportantfactorin
the ability of the latter to produce disease and in the speed with which a primary
lesion developed. He seems to have based this conclusion upon the fact that he
obtained a higher percentage of positive inoculations in apes inoculated with
material from human sources containing large numbers of treponemata than in
animals inoculated with material containing smaller numbers of organisms.
We have not been able to find in his communications any protocols of experiments
in which he used graduated doses of the same emulsion.
'Neisser, A., Arb. k. Gsndhtsamte, 1911, xxxvii, 1.
479
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The purpose of this communication is to record the results of an
experiment which was designed to ascertain whether or not the size
of the inoculum exerts any effect upon the course of experimental
syphilis in the rabbit. The behavior of several syphilitic rabbits
observed during the course of some other experiments had suggested
the possibility that the course of this infection in rabbits, particularly
the incidence and character of generalised lesions, might be dependent in part upon the number of treponemata in the inoculated material. Moreover, it seemed desirable to ascertain the influence of this
factor upon the incubation period, since much stress has been laid upon
the duration of the latter in attempts by several workers to differentiate various strains of Treponema pallidum.
Published April 1, 1925
480
EXPERIMENTAL SYPHILIS.
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Published April 1, 1925
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Published April 1, 1925
482
EXPERIMENTAL SYPHILIS.
I
EXPERIMENTAL.
RESULTS.
For lack of space the individual protocols have been omitted and
the results will be presented in tabular form. The particular points
studied were (1) the incubation period, (2) the duration of the initial
orchitis before there was clinical evidence of regression of the same,
(3) the occurrence of scrotal edema in the inoculated testicle, (4) the
occurrence of a scrotal chancre in the inoculated testicle, (5) the
occurrence of orchitis or epididymitis in the uninoculated testicle,
and (6) the occurrence of generalised lesions elsewhere. The incidence of these various conditions is set forth in Table I.
Analysis of Table I brings out the following facts.
Incubation Period.-The average incubation period of the group
receiving the largest inoculum was least, while that of the group receiving the smallest inoculum was greatest. While there were a
few exceptions to this rule they were not sufficient to modify it appreciably. Under the conditions of the experiment there was an
inverse ratio between size of inoculum and incubation period, as one
might expect. The difference in the average incubation period between Groups A and B was 5.8 days, that between Groups B and C,
4 days, and that between Groups A and C, 9.8 days.
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Three groups of adult male rabbits comprising individuals of varying breeds
were utilised for the experiment. The Nichols strain of Treponema pallidum was
selected because of its high degree of virulence for the rabbit and because its
behavior in this animal is better known perhaps than that of any other American
strain. All inoculations were made into the right testicle and the animals were
kept under observation for a period of 192 days.
The individual members of each group received the same amount of inoculum.
The material used for inoculation consisted of mixed testicular emulsions from two
rabbits in which the testicular lesions were comparatively fresh and were increasing in extent at the time the emulsions were prepared. The mixed emulsion contained five to ten actively motile treponemata per field and constituted, therefore,
a rich inoculum. Of this emulsion, Group A, comprising six rabbits, received 0.8
cc. each, Group B of five rabbits received 0.2 cc. each, and Group C of six rabbits
received 0.05 cc. each. These dilutions were selected because they represent about
the limits of dosage that are ordinarily employed in this type of work. The
inoculation was made into the body of the right testicle in each rabbit.
Published April 1, 1925
ALAN M. CHESNEY AND JAROLD E. KEMP
483
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Duration of Initial Orchitis.-The average duration of the initial
orchitis reckoned from the appearance of the first clinical sign of infection until there was definite clinical evidence of beginning regression of the lesion did not vary appreciably in the various groups,
nor was there any relationship observable between this factor and
that of size of inoculum. The determination of the exact time at
which regression of a syphilitic orchitis sets in is, at best, somewhat
uncertain, and such differences as were encountered in this respect
in the three groups would come well within the experimental error.
The extremes of duration of the initial orchitis were encountered in
each of the three groups.
Scrotal Edema.-This type of reaction was encountered in the majority of the animals in the two groups receiving the largest inocula.
It was encountered only once in the group receiving the smallest
inoculum.
Scrotal Chancres.-The incidence of scrotal chancres on the inoculated side was essentially the same in all three groups. In one
animal receiving the smallest size inoculum two chancres developed
simultaneously.
Metastatic Orchitis and Epididymitis.-The occurrence of orchitis
or epididymitis in the uninoculated testicle was much less in the group
receiving the smallest inoculum than in the other two groups, the incidence being once in six animals, or 16.6 per cent.
Metastatic Skin, Bone, and Eye Lesions.-The percentage of animals showing metastatic lesions other than orchitis or epididymitis
was greatest in the group receiving the largest inoculum (66.6 per cent)
and least in the group receiving the smallest inoculum (50 per cent).
The difference between these two groups amounted to only 16.6 per
cent, however. It should be noted at this point that the animal which
showed the greatest number of metastatic lesions in the entire series
was one which received the smallest inoculum. This animal, No.
17, presented the appearance of malignant syphilis, having at one
time no less than four skin lesions, bilateral keratitis, and five separate scrotal chancres, two on the inoculated side and three on the
opposite side.
This experiment may be said not to have fulfilled all of the expectations that were in mind at the time it was planned, since, as has
Published April 1, 1925
484
EXPERIMENTAL SYPHILIS.
I
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already been stated, the behavior of several animals observed in the
past had led us to believe that perhaps the size of the inoculum might
exert some influence upon the subsequent course of the disease. On
the whole, it was impossible to obtain any clear-cut evidence that the
character of the disease was affected by the dosage of inoculum. It
is true that the incidence of scrotal edema was considerably less in
the group of animals receiving the smallest inoculum, and the initial
reaction on that account was not so impressive as in the other groups,
nor was the incidence of metastatic orchitis so great. However,
these factors constitute only a portion of the picture of experimental
syphilis in the rabbit and should not in our opinion receive undue
consideration.
There was an appreciable and regular difference in the incubation
period dependent upon the size of the inoculum. As one would expect a priori, the incubation period was shortest in the group receiving the largest inoculum and became progressively longer as the
dosageof infecting material was diminished. In this respect the experiment bears out conclusively the views of Neisser regarding the influence of quantity of inoculum upon the duration of the incubation
period in experimental syphilis in apes.
On the whole, the impression which was gained from this study was
that while it is possible, by increasing the number of treponemata
inoculated in the testicle, to speed up the initial reaction and render
it more impressive by the occurrence of a massive scrotal edema, still
the general picture of the disease is but little altered by varying the
size of the inoculum within the limits utilised in the experiment. It
must be borne in mind, however, that even the smallest inoculum
which was used was a comparatively rich one, and the strain was one
which is highly pathogenic for rabbits. It is entirely possible that
with less virulent strains or with even smaller inocula of a virulent
strain greater differences in the course of the experimental disease
might be brought out.
The behavior of one rabbit, in which the disease ran a particularly
malignant course after a minimal amount of virus was inoculated,
illustrates the principle that malignant syphilis is the expression of
an unusual degree of susceptibility of the host to the syphilitic virus
rather than an indication of exalted virulence or maximal amount of
inoculum.
Published April 1, 1925
ALAN M. CHESNEY AND JAROLD E. KEMP
485
CONCLUSIONS.
1. The size of the dose of the infecting material exerts an appreciable effect upon the duration of the incubation period in experimental
syphilis of the rabbit. Other things being equal, the larger the dose
the shorter is the incubation period.
2. The size of the inoculum was not found to exercise any marked
effect upon the course of the disease as a whole.
3. In individual rabbits inoculated with minimal amounts of virus
the disease may run a malignant course.
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